SYMBIOSIS - Cyanobacteria - page 363

their hydropathy patterns and soluble characteristics. The former are present in both ascomycetous
and basidiomycetous fungi whereas the latter have been detected only in ascomycetous fungi. Class
II hydrophobins are soluble in hot SDS solution and those of class I dissociate into monomers upon
incubation in trifl uoroacetic acid and performic acid (de Vries et al. , 1993; Wessels et al. , 1991).
Structurally, hydrophobins of both the classes are about 100 amino acids long and contain eight
conserved cysteine residues (Wessels, 1994, 1997). The cysteine residues are involved in intramolecular
cross-linkages by forming disulphide bonds with each other. Studies on cerato-ulmin indicated that
these disulphide bridges are formed with probable linkage of Cys1-Cys2, Cys3-Cys4, Cys5-Cys6 and
Cys7-Cys-8. Hydrophobins once produced in the fungal cell are secreted to outside via ER-Golgi
pathway. These contain at their N-terminal part a signal peptide consisting of a few amino acids. It is
speculated that this signal peptide serves as an anchor to the hydrophobins to the outside of the fungal
cell walls. The property of self assembly is retained even after the disulphide bridges are reduced
and the sulphydryl groups are blocked with iodoacetamide, although the resulting proteins were
unable to re-form disulphide linkages. It is suggested that the disulphide bonds inhibit premature
self-assembly of the hydrophobins prior to their secretion into the wall (deVocht et al ., 2000). The
polyhydrophobins of Claviceps spp. are 400 amino acids long. Those from C . fusiformis , CFTH1 are
tripartite hydrophobins, i.e. three class II bimodular hydrophobins are encoded as a single protein
by a multimeric gene. However, the physical and chemical properties of CFTH1 resemble those of
class II hydrophobins. A penta hydrophobin has also been described from C . purpurea (Whiteford
and Spanu, 2002).
Hydrophobins of class I from Schizophyllum commune are the best studied and the growing fungal
tips secrete SC3 monomers that assemble spontaneously at hydrophilic/hydrophobic interfaces, i.e.
between water and air, water and oil, or water and hydrophobic solid like tefl on into an amphipathic
fi lm/membrane (Wösten et al ., 1993, 1994a,b, 1995). The hydrophilic side of the membrane orients
and attaches itself to the cell wall while the hydrophobic side gets exposed to the hydrophobic
environment. Thus aerial hyphae and spores tend to become aerophobic and hyphae that grow over
hydrophilic substrata get themselves attached to the surface (Wösten, 2001).
The capacity of lichens to withstand repeated cycles of desiccation and fl ooding enables them to
colonize extreme habitats. The inner cavities of lichens are lined with hydrophobic membrane. This is
also known as rodlet layer. The isolation and cloning of genes and localization of hydrophobins has
now been achieved both from ascomycetous and basidiomycetous fungi that form the lichen thalli.
Honegger (1991) proposed two major functions to the rodlet layer. The fi rst is that at the immediate
contact site of the growing hyphae with the cell wall surface of algal cells, the hydrophobins diffuse
and spread over the surface of the photobiont. Thus a continuous surface hydrophobic layer is
formed at wall-air interfaces of both the partners. Secondly, it prevents accumulation of water in
the interior of the thallus and helps in the supply of optimal fl ow of water and solutes from the
exterior to the algal layer and vice versa. This has also received much support from other workers
(Wessels, 2000; Dyer, 2002).
The fi rst report on the occurrence of hydrophobins in lichen-forming (ascomycetous) fungi Xanthoria
parietina and X . ectaneoides was made by Scherrer et al . (2000) who demonstrated that hydrophobins
of class I designated as XPH1 and XEH1, respectively produced by these fungi are of the size of 10
kDa. These assembled in vitro as shown by transmission electron microscope studies into a rodlet
layer with individual rodlets of ~10nm. This was further confi rmed by the fact that the antibodies
raised against hydrophobins are bound to this structure (Scherrer et al ., 2002; Trembley et al ., 2002a).
A common feature of X . parietina and Dictyonema glabratum is the presence of gas-fi lled spaces in the
photobiont layers. Despite the fact that the thalli of D . glabratum daily undergo the cycles of hydration